Sensors with microscopic or nanoscopic elastic elements are known from the prior art allowing for measuring the smallest forces or resonances up to the GHz range.
If the elastic elements are constructed as resonators, they can be used as a mass detector. The detection principle is based on the detection of a shift of the resonant frequency when particles are deposited on the resonator. The extent of the shift is a measure for the mass of the deposited particle. Due to the extremely small dimensions of the currently producible resonators, even the smallest mass deposits lead to a measurable shift of the resonant frequency, so that by now even the depositing of individual atoms can be measured, cf. “An atomic-resolution nanomechanical mass sensor”, K. Jensen, Nature Nanotechnology, 3, 533, 2008.
However, suitable reading methods are required for sensors of this type, by which the deflection of the elastic elements can be measured. For resonators clamped on one side, as are used for example in the atomic force microscope (AFM), the deflection of a laser beam in total reflection from the AFM bar, the so-called cantilever, is measured. This type of detection of the deflection is here termed “external deflection”, as the detection device, i.e. the illumination source and the optical detector for receiving the reflected light, is structurally separated from the actual sensor.
Alternatively, so-called internal detection methods are known, which are also termed “on-chip” methods and in which an integrated tension sensor on the cantilever detects the movement or deformation of the cantilever, cf. “A Review of Microcantilevers for Sensing Applications”, S. Vashist, A. Zojono, 2007, Medical News TODAY, Jun. 24, 2007.
The dimensions of conventional cantilevers lie in the micrometer range. However, if the sensor is to be reduced in size to the nanometer range, the reflection measurement of the laser beam is no longer practically possible on the cantilever, and the only remaining option for the person skilled in the art is electronic on-chip detection, which necessitates complex and expensive reading electronics. However, this must result in the surrendering of a significant advantage of external detection, namely that the sensors, which are subject to wear, can be exchanged simply in the case of a design with external detection, while the detection device itself can be retained.
Cell force sensors are a further class of sensors in which the deflection of microscopic or nanoscopic elastic elements is detected and by use of which the forces exerted by biological cells shall be measured. A force sensor of this type is known from Olivia du Roure et al., “Force mapping in epithelial cell migration”, Proceedings of the National Academy of Sciences of the United States of America, 102(7):2390-2395, 2005. The force sensor comprises a multitude of vertical columns which are fastened with a lower end on a horizontal substrate and have an upper free end. The columns and the substrate consist of polydimethylsiloxane (PDMS), i.e. an elastomeric material. During the force measurements, the cells are located on the surfaces of a plurality of adjacent columns. The forces exerted by the cells lead to a deflection of the columns out of their rest positions, which can be detected optically. In the case of a known spring constant of the columns, it is then possible to deduce the cell force from the deflection.
Also with this type of sensor, the currently known type of detection of the deflection of the elastic elements places limits on miniaturisation, as the optical resolution of a typically used microscope is limited by diffraction effects.
Furthermore, it is barely possible from a practical point of view to detect the cell forces of more than a few individual cells at the same time using current force sensors. This means that with currently available methods and devices, it is very complex to obtain significant statistics about cell forces.
In summary, it is therefore seen that in the case of both the described mass sensors and force sensors, the detection of the deflection of the elastic elements becomes more difficult, the more the elastic elements are miniaturised and the greater the number of elastic elements is, whose deflection shall be detected.